This invention relates to a communication apparatus and a communication method using multi-carrier transmission system, particularly, to a communication apparatus and a communication method which used a multi-carrier transmission system (Digital Wavelet Multi Carrier transmission system, hereinafter, referred to as DWMC transmission system) which carries out data transmission by digital modulation and demodulation processing which used a real coefficient wavelet filter bank which is more suitable for a power line transmission channel and a transmission channel such as a telephone line.
As a conventional technology which has been frequently used in the multi-carrier transmission system, there are FFT (Fast Fourier Transform) based OFDM (Orthogonal Frequency Division Multiplexing) and Wavelet based OFDM. Such an example that these technologies were applied to power line communication is disclosed in (JP-A-11-163807). The wavelet base has a resistance property to inter-carrier interference since a side Lobe of an amplitude spectrum is low, and is of an excellent characteristic. In addition, in order to avoid collapse of orthogonality, in the FFT based OFDM, GI (guard interval) is indispensable, while in the wavelet based OFDM, it is not necessary. This improves transmission efficiency. Since processing of the FFT based OFDM is known well, an explanation will be omitted. Since the wavelet based OFDM is digital modulation and demodulation processing which used a real coefficient wavelet filter bank, it is a multi-carrier system of a kind, and is a thing which generates a transmission signal by combining a plurality of digital modulation waves by the real coefficient filter bank. As a modulation system of each carrier, PAM (Pulse Amplitude Modulation) is used. Data transmission by a DWMC transmission system is transmitted in such a manner that an impulse response of each sub carrier is overlapped in each sub carrier as shown in FIG. 20. Each transmission symbol becomes such a time wave that an impulse response of each sub carrier was combined as shown in FIG. 21. An example of the amplitude spectrum is shown in FIG. 22. In the DWMC transmission system, approximately several hundred transmission symbols of FIG. 20 are collected to configure one transmission frame. A configuration example of a DWMC transmission frame is shown in FIG. 23. In this DWMC transmission frame, in addition to an information data transmission symbol, a preamble symbol etc., which are used for carrier detection, synchronization, equalization and so on, are included. A conceptual configuration of a power line communication apparatus in case that the DWMC transmission system was adopted is shown in FIG. 19. Firstly, in a transmitting device 299, bit data is converted into symbol data by a symbol mapper 210, and in accordance with each symbol data, symbol mapping (PAM) is carried out. And, in a serial-parallel converter 220, a real value di (i=1˜M) is given with respect to each sub carrier, and in an inverse wavelet transformation part 230, inverse wavelet transformation is carried out on a time axis. By this, a sample value of a time axis waveform is generated, and a sample value sequence, which represents a transmission symbol, is generated. In a D/A converter 240, it is converted from this sample value sequence to a base band analog signal waveform, which is continued in terms of time, and transmitted. In a receiving device 399, a reception signal is converted into a digital signal in an AID converter 310, and wavelet-transformed so as to be able to handle phase information in a complex wavelet transformer 320, and in a carrier detector 330, presence or absence of the reception signal is detected, and in a synchronous circuit 340, synchronizing timing is extracted from the reception signal, and in an equalizer 350, the reception signal is compensated so as to cancel influence of a transmission channel, and in a channel estimation unit 370, a state of a power line transmission channel is estimated, and in a decision unit 380, the reception signal is decided by use of a threshold level. Here, the number of pieces of the sample values on a time axis, which are generated by inverse wavelet transform, is normally 2n (n is a positive integer) pieces.
In the meantime, in a conventional system, there was such a problem that transmission channel estimation, which is carried out in a receiving device, cannot follow sufficiently to instantaneous fluctuation and periodical fluctuation of wide-band noise or narrow-band noise, or instantaneous fluctuation and periodical fluctuation which come up with amplitude fluctuation and phase fluctuation of a transmission channel itself, when transmission channel estimation is simply carried out only once in a certain cycle, in a power line transmission channel. Here, as one example of the power line transmission channel, an attenuation characteristic of the power line transmission channel is shown in FIG. 24. In addition, FIG. 25 is a view which shows a group delay characteristic of the power line transmission channel.
As described above, in a multi-carrier power line communication apparatus which used the conventional wavelet, it has such a problem that transmission channel estimation, which is carried out in a receiving device, cannot follow sufficiently to instantaneous fluctuation and periodical fluctuation of wide-band noise or narrow-band noise, or instantaneous fluctuation and periodical fluctuation which come up with amplitude fluctuation and phase fluctuation of a transmission channel itself, when transmission channel estimation is simply carried out only once in a certain cycle, in a power line transmission channel. In this power line communication apparatus, it is requested to sufficiently take hold of a transmission channel state, and to heighten transmission efficiency.